Component parts inside a chamber of plasma etching equipment or the like in a semiconductor manufacturing process are exposed to corrosive environments with a corrosive gas. When the corrosive gas is activated by plasma, a corrosion phenomenon becomes more prominent.
On a surface of the component parts exposed to the corrosive gas, a reaction product between the component parts and the corrosive gas is formed. As a result of this reaction, the component parts are corroded and change in their shapes. Finally, it becomes impossible to keep their shapes as designed.
During the formation of the reaction product, vaporization, volatilization and flaking of the reaction product occur. Consequently, particles are formed in the chamber, which cause contamination of an inside of the chamber and an etching target object (particularly, a semiconductor wafer). If such particles adhere to the etching target object, the defects occur, such as insulation defects or shape defects. It could be a factor of hindering yield enhancement in a semiconductor manufacturing process.
For example, aluminum, aluminum alloy, alumited aluminum (aluminum subjected to an alumite treatment), aluminum oxide (Al2O3) or aluminum nitride (AIN) has heretofore been used as a material for a component parts to be exposed to a corrosive gas or a plasma treatment using a corrosive gas. However, these materials do not have sufficient resistance to corrosion. Therefore, an improved high corrosion resistant material is being required in order to improve the quality and yield enhancement in the semiconductor manufacturing process.
Currently, yttrium oxide (Y2O3) and yttrium aluminum garnet (YAG) are drawing attention because of their excellent resistance to corrosion as compared to the above mentioned aluminum alloy and aluminum oxide. However, Y2O3 and YAG-based ceramic materials have difficulty in obtaining a dense sintered body due to its poor sinterability. Moreover, they have low mechanical strength (strength, hardness). For these reasons, Y2O3 and YAG-based ceramic materials have been scarcely put to practical use. Further, Y2O3 and YAG-based ceramic materials which contain a large amount of rare-earth element are costly as compared to other ceramic materials. An efficient improvement for cost reduction is necessary to realize practical applications of Y2O3 or YAG-based ceramic material.
Magnesium oxide (MgO) also has great potential as a corrosion resistant material, because it is excellent in resistance to corrosion as compared to the above aluminum alloy and aluminum oxide. Depending on corrosive conditions, magnesium oxide has higher resistance to corrosion than those of the above yttrium oxide and yttrium aluminum garnet. Further, the element magnesium has the 8th highest Clarke number, and thereby magnesium oxide is extremely low in cost. Therefore, adopting magnesium oxide can contribute to improve corrosion resistance and cost reduction. In addition, magnesium oxide has higher thermal conductivity than those of the above aluminum oxide, yttrium oxide and yttrium aluminum garnet. Their feature of the high thermal conductivity is useful in the process with high-temperature treatment and in the process requiring a uniform heating ability.
On the other hand, it is difficult to obtain a dense sintered body of magnesium oxide. Moreover, magnesium oxide has hardness (Vickers hardness) of about 550 Hv and a bending strength of about 250 MPa, even in the form of a dense sintered body. These physical property values are particularly low among structural ceramic materials. In order to allow a magnesium oxide-based ceramic material to be used for various component parts like component parts for semiconductor manufacturing equipment, it is necessary to improve mechanical property.
Heretofore, there have been various proposals which can improve their sinterability and mechanical strength of a magnesium oxide-based ceramic material.
As one example, the following Patent Document 1 discloses a ceramic material which contains 5 to 95 weight % of magnesium oxide, with the remainder being rare-earth element-containing oxide or composite oxide. In the example of the specification, the Patent Document 1 also discloses a composite ceramic material which contains magnesium oxide, with the remainder being yttrium oxide or YAG.
However, in the Patent Document 1, when the remainder is yttrium oxide, a resulting composite ceramic material becomes insufficient in hardness. This is because both of magnesium oxide and yttrium oxide are low in hardness. Moreover, the composite ceramic material consisting of magnesium oxide and yttrium oxide has difficulty in densification by sintering, and exhibits low bending strength.
In the Patent Document 1, in order to allow the composite ceramic material consisting of magnesium oxide and rare-earth-containing composite oxide such as YAG to have improved mechanical properties, it is necessary to increase an amount of the rare-earth-containing composite oxide. However, this causes a deterioration in resistance to corrosion, a lowering in thermal conductivity and an increase in manufacturing cost of a resulting composite ceramic material. Therefore, it is desired to maintain the mechanical properties while reducing the amount of the rare-earth-containing composite oxide.
The following Patent Document 2 discloses a composite ceramic material which consists substantially of MgO, Al2O3, and ZrO2 and/or Y2O3, wherein a composition ratio of MgO to Al2O3 by weight ratio is set in the range of 0.67 to 2.33, and ZrO2 and/or Y2O3 are contained in a total amount of 1 to 10 weight %.
However, in the composite ceramic material disclosed in the Patent Document 2, a resistance to corrosion of Al2O3 and ZrO2 is largely inferior to that of MgO. Thus, an Al2O3 phase and a ZrO2 phase are selectively corroded at an early stage. Moreover, ZrO2 is transformed along with a volume change caused by a temperature rise. Therefore, a ZrO2-containing ceramic body is easily broken due to a temperature change.
The Patent Document 2 also discloses a ceramic material containing no ZrO2. However, in the case where the composite ceramic material in the Patent Document 2 contains no ZrO2, hardness and bending strength are lowered. Specifically, the hardness and bending strength become less than those of the conventional aluminum oxide-based or aluminum nitride-based material used for component parts for a semiconductor manufacturing equipment.
In the specification including Example, the Patent Document 2 mentions that the Al2O3 may be replaced with spinel (MgAl2O4). Spinel has higher resistance to corrosion than that of alumina. However, even this replacement cannot compensate for an insufficiency in hardness of Y2O3 used for improving sinterability and strength, and an insufficiency in resistance to corrosion of ZrO2. Moreover, the composition devoid of ZrO2 is low in mechanical properties.
As above, conventional ceramic materials comprising Mg, Al, Y and O are as disclosed in the Patent Documents 1 and 2. However, in the Patent Documents, a reaction in high-temperature (sintering) conditions is not considered. The Patent Documents simply disclose producing a ceramic material comprising Mg, Al, Y and O, but scarcely discloses physical properties and a composition after burning.
An actual sintered body of a ceramic material comprising Mg, Al, Y and O can take various composites depending on a ratio between the elements. Although the Patent Document 2 mentions that aluminum oxide and magnesium oxide react with each other to form spinel, it makes no mention of other reaction, particularly, a reaction of rare-earth oxide. For example, in a sintered body obtained by mixing magnesium oxide, aluminum oxide and YAG, and sintering the mixture, YAG can actually remain only on a condition that the mixture is set in a limited composition range. This means that a ceramic material comprising Mg, Al, Y and O is formed as totally different products (sintered bodies) depending on a ratio between the elements and an amount of oxygen.
Generally, an oxide-based ceramic body including a magnesium oxide-based ceramic body, a nitride-based ceramic body including an aluminum nitride-based ceramic body and the like is an electrical insulating body. However, component parts to be used inside a semiconductor manufacturing equipment, such as electrostatic chucks, ring shaped parts, shower heads and chambers, are required to have a low electrical resistivity, in some cases. This is because when an electrical insulating ceramic body is used as each of the above component parts, a surface of the component parts is electrostatically charged, so that the aforementioned reaction product is more likely to adhere to the surface of the component parts. If the reaction product flakes off from the component parts and drops on a semiconductor wafer, the semiconductor wafer becomes defective. Moreover, the electrostatic charge of the component parts causes abnormal electrical discharge. In contrast, when the component parts have a low electrical resistivity, it becomes free of electrostatic charge, thereby preventing the occurrence of the above problems.
Further, the component parts to be used within the semiconductor manufacturing equipment, such as electrostatic chucks, ring shaped parts, shower heads and chambers, are different from each other in terms of a required electrical resistivity. Thus, a conventional electrical insulating ceramic body having an electrical resistivity of 1015 Ω·cm or more is likely to fail to obtain sufficient properties. That is, it is necessary to adjust an electrical resistivity of a ceramic body for each intended use.